The Securities and Exchange Commission yesterday ordered public hearings into charges that First Jersey Securities Inc.-a fast-growing Wall Street frim with offices in the Washington area-has fraudulently been buying up cheap stock and peddling it to unsuspecting investors at inflated prices.

Citing the results of a three-year investigation, the SEC alleged that First Jersey had deceived thousands of investors into purchasing stocks at prices much higher that First Jersey had paid for the shares only a short time before. The markups ranged as high as 150 percent the SEC said.

In addition to stock manipulation, the SEC accused the four-year-old company-along with its president and seven of its current and former employes-of making false and incomplete statements about stocks it sold, misrepresenting its research abilities to investors, failing to file the required disclosure forms and keeping inaccurate corporate books and records.

First Jersey President Robert Brennan issued a statement calling the allegations "absolutely outrageous and completely untrue" and vowing a spirited defense.

"The SEC's action is just one more example of government interference in and oppression of honest business," Brennan said. "It is ironic that, at a time when the SEC is supposedly doing all it can to encourage financing of smaller companies, it initiates an action against the one firm in Wall Street which has consistently and successfully provided capital markets and liquidity for smaller companies and their public shareholders," Brennan said in his statement.

First Jersey specializes in stocks that are not the blue chip variety found listed on the two major exchanges. It sells stocks traded over the counter (OTC), a vast marketplace for the stocks and bonds of about 30,000 companies.

The company, created in 1974, had sales of $12.5 millin last year - a jump of 300 percent just over the year before. The details of how First Jersey operates were described in a Washington Post article Sunday.

What emerged was a portrait of First Jersey as one of the most successful "boiler room" operations in recent history, where salesmen, operating from 15 offices in eight states and equipped only with a desk and a telephone, earn big commissions for each share of stock they can persuade a customer to buy.

The SEC cited several instances in which First Jersey, as a result of domination and control over some stocks, was able to set an arbitrary price and gain a substantial profit:

In April 1975, the company sold 160,000 shares of Glenco Scientific Inc.at $4.53 per share which relatives of Brennan's had bought one or two months before at a cost of 75 cents per share.

Between March and May of 1975, the company sold thousdands of shares of URT Industries Inc. at from $1 to $2 5/8 per share, considerably more than the 48 cents per share it had paid for the stock in March.

From November 1977 to January 1978, the company sold 3.5 million shares of Sequential Information Systems Inc. at prices from 24 to 62 1/2 cents per share. Brennan had acquired the stock at no more than 9 cents per share.